Pulsed gas delivery systems are commonly used in the formation of multi-layer thin films on a batch of substrates located in a process chamber. One such technique for forming thin films on substrates is atomic layer deposition (ALD), in which gaseous reactants, or “precursors”, are sequentially delivered to a process chamber to form very thin layers, usually on an atomic-layer scale, of materials on the substrates.
By way of example, a high dielectric constant capacitor may be formed on a silicon wafer using an ALD technique through the sequential deposition of hafnium oxide (HfO2) and aluminium oxide (Al2O3) thin films. HfO2 thin films may be formed by the sequential supply of a hafnium precursor, such as tetrakis(ethylmethylamino)hafnium (TEMAH), and an oxidant, such as ozone (O3), to the process chamber, and Al2O3 thin films may be formed by the sequential supply of an aluminium precursor, such as trimethyl aluminium (TMA), and O3 to the chamber.
In overview, the first precursor delivered to the process chamber is adsorbed on to the surfaces of the substrates within the process chamber. The non-adsorbed first precursor is drawn from the process chamber by a vacuum pumping system, and the second precursor is then delivered to the process chamber for reaction with the first precursor to form a layer of deposited material. In the deposition chamber, the conditions immediate to the substrates are optimised to minimise gas-phase reactions and maximise surface reactions for the formation of a continuous film on each substrate. Any non-reacted second precursor and any by-products from the reaction between the precursors is then removed from the process chamber by the pumping system. Depending on the structure being formed within the process chamber, the first precursor, or a third precursor, is then delivered to the process chamber.
A purge step is typically carried out between the delivery of each precursor, for example by delivering a purge gas, such as N2 or Ar, to the chamber between the delivery of each precursor. The purpose of the purge gas delivery is to remove any residual precursor from the process chamber so as to prevent unwanted reaction with the next precursor supplied to the chamber.
In practice, only around 5% or less of the precursors supplied to the process chamber are consumed during the deposition process, and so the gas drawn from the chamber during the process chamber will, between supplies of purge gas to the chamber, alternately be rich in the first precursor, and then rich in the second precursor.
In convention vacuum pumping systems, the gases drawn from the process chamber enter a common foreline leading to a vacuum pump. In the event that the non-reacted precursors meet within the vacuum pumping system, cross-reaction of the precursors can occur, and this can result in both the deposition of solid material and the accumulation of powders within the foreline and the vacuum pump. Particulates and powders that have accumulated within the pump can effectively fill the vacant running clearance between the rotor and stator elements of the pump, leading to a loss of pumping performance and ultimately pump failure. Periodic pump cleaning or replacement is then required to maintain pumping performance, resulting in costly process downtime and increasing manufacturing costs.
In order to avoid reaction between non-reacted precursors downstream from an ALD process chamber, US 2004/0107897 describes a vacuum pumping system in which the foreline from the process chamber branches into first and second exhaust lines each leading to a respective vacuum pump. A first exhaust control valve is installed within the first exhaust line, and a second exhaust control valve is installed within the second exhaust line. The opening and closing of the control valves is timed in relation to the supply of first and second precursors to the process chamber so that during a period in which the first precursor is being supplied to the process chamber, the second exhaust line is closed, and during a period in which the second precursor is being supplied to the process chamber, the first exhaust line is closed. This reduces the extent of the mixing of the non-reacted precursors within the exhaust lines and the vacuum pumps, increasing process uptime and throughput.
A problem associated with both the conventional vacuum pumping system and the vacuum pumping system described in US 2004/0107897 is that if the, or one of the, vacuum pumps fails during processing within the process chamber, the batch of substrates currently being processed within the chamber will be lost. In order to increase product throughput, the numbers of substrates being processed within a process chamber at any one time may be anywhere between 25 and 100 or more, and so failure of a vacuum pump during a deposition process may be very costly.